I read on yahoo news it wasn't the real Arnold. It was just a stunt double with a real close resemblance and in some bits they sort of digitally enhanced Arnold's face [if that's the right way to discribe it] onto the T800 face.
"For the T-800 scene, in which John Connor (Christian Bale) confronts the old nemesis, Austrian bodybuilder Roland Kickinger played the Terminator so that Bale would have someone to act opposite. Schwarzenegger's face was superimposed on Kickinger's head in post-production."
If Schwarzenegger had not agreed to allow the use of his face and likeness for the T-800, McG had a backup plan. "You'll notice the door flies off, and Connor goes down on his back, and he shoots the machine gun up the chest of the T-800," McG said. "If we were unsuccessful in getting the likeness of Schwarzenegger, we were just going to have the machine gun having blown his face off. It would have been the endoskeleton, the skull idea,
and it wouldn't have been nearly as satisfactory. In the end, I think the governor's very pleased with what we did and the manner in which it was handled. So I think that's a nice moment in the movie, when that shows up."
That's for damn sure. :wiggle:
http://scifiwire.com/2009/05/how-mcg-made-that-t-800-a.php